A seismic survey represents an attempt to image or map the subsurface of the earth by sending energy down into the ground and recording the “echoes” that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, or air guns in marine environments. During a seismic survey, the energy source is placed at various locations near the surface of the earth above a geologic structure of interest. Each time the source is activated, it generates a seismic signal that travels downward through the earth, is reflected, and, upon its return, is recorded at a great many locations on the surface. Multiple source/recording combinations are then combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2D) seismic survey, the recording locations are generally laid out along a single line, whereas in a three dimensional (3D) survey the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3D survey produces a data “cube” or volume that is, at least conceptually, a 3D picture of the subsurface that lies beneath the survey area. In reality, though, both 2D and 3D surveys interrogate some volume of earth lying beneath the area covered by the survey.
A conventional seismic survey is composed of a very large number of individual seismic recordings or traces. In a typical 2D survey, there will usually be several tens of thousands of traces, whereas in a 3D survey the number of individual traces may run into the multiple millions of traces. Chapter 1, pages 9-89, of Seismic Data Processing by Ozdogan Yilmaz, Society of Exploration Geophysicists, 1987, contains general information relating to conventional 2D processing and that disclosure is incorporated herein by reference. General background information pertaining to 3D data acquisition and processing may be found in Chapter 6, pages 384-427, of Yilmaz, the disclosure of which is also incorporated herein by reference.
A seismic trace is a digital recording of the acoustic energy reflecting from inhomogeneities or discontinuities in the subsurface, a partial reflection occurring each time there is a change in the elastic properties of the subsurface materials. The digital samples are usually acquired at 0.002 second (2 millisecond or “ms”) intervals, although 4 millisecond and 1 millisecond sampling intervals are also common. Each discrete sample in a conventional digital seismic trace is associated with a travel time, and in the case of reflected energy, a two-way travel time from the source to the reflector and back to the surface again, assuming, of course, that the source and receiver are both located on the surface. Further, the surface location of every trace in a seismic survey is carefully tracked. This allows the seismic information contained within the traces to be later correlated with specific surface and subsurface locations, thereby providing a means for posting and contouring seismic data—and attributes extracted therefrom—on a map (i.e., “mapping”).
Many variations of the conventional source-receiver arrangement are used in practice, e.g. VSP (vertical seismic profile) surveys, ocean bottom surveys, etc. Further, the surface location of every trace in a seismic survey is carefully tracked and is generally made a part of the trace itself (e.g., as part of the trace header information). This allows the seismic information contained within the traces to be later correlated with specific surface and subsurface locations, thereby providing a means for posting and contouring seismic data—and attributes extracted therefrom—on a map (i.e., “mapping”).
For all of the subsurface information that might be acquired via seismic data, this method is not without its problems. For example, one particularly troublesome problem in seismic data collection and analysis is the identification and removal of multiples. Those of ordinary skill in the art will understand that multiples in seismic data occur when the recorded seismic data contain energy that has been reflected more than once in the subsurface. Multiples often appear to all intents and purposes to be valid seismic reflectors and, to the extent that they are interpreted as such, can give rise to incorrect interpretations of the subsurface layer configuration, thereby potentially resulting in the drilling of dry holes.
One method of attenuating multiples that has had some success is known as “surface-related multiple elimination” or “SRME”, hereinafter. In brief, this method operates by creating a dataset that contains only predictions of the multiples that are present in the data. Specifically, the method seeks to predict the seismic expression of multiples that have experienced one or more reflections between, for example, the air-water interface and the subsurface. Then, the predicted multiples are subtracted from the original data leaving behind (at least theoretically) only the primary energy.
The application of SRME has provided significant added value to practical processing projects over the last decade, yet the promise of a true 3D solution has been elusive. It has long been known that full 3D SRME multiple prediction is best applied to data coverage with unaliased source and receiver sampling over a full range of azimuths and offsets which is only roughly approximated by the conventional narrow azimuth towed streamer (NATS) 3D seismic data typically available today. In more particular, and has been observed in a variety of different contexts, in order to accurately image complex subsurface structures the structure should be illuminated from a variety of different offsets and azimuths. Wide azimuth surveys have been done for many years and have often proven to yield superior data that can be subsequently migrated or otherwise imaged to produce an improved picture of the subsurface as compared with traditional/narrow azimuth surveys. Where the survey is not designed to acquire wide azimuth information, methodologies designed to estimate the required wide azimuth seismic data from narrow azimuth data have improved the ability to predict complicated out-of-plane multiples (e.g., diffractions) to a limited degree.
Conventional methods for predicting and attenuating surface-related multiples either assume that the subsurface is 2D and thus are applicable only to 2D data shot in the dip direction or they assume that wide azimuth seismic data can be adequately extrapolated from narrow azimuth data to enable a 3D multiple prediction.
Heretofore, as is well known in the geophysical prospecting and interpretation arts, there has been a need for a method of using seismic data to obtain image of the subsurface that does not suffer from the limitations of the prior art. Accordingly, it should now be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for a method of geophysical prospecting that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.